5.1 COMMUNICATION NETWORKS TELECOMMUNICATIONS Telecommunications began with the invention of the electric telegraph in the 19th century, soon becoming one of the fastest growing fields of industry. Telecommunications over telephone lines is also called point-to-point because it is between one transmitter and one receiver. megaphone: megafono transceiver: ricetrasmittente transducer: trasduttore 204 LINKING COMPUTERS Communication and telecommunications Communicating is the process of sending information from one place to another. When this process of transmission involves significant distances, it is called telecommunications . The objective of telecommunications is to convert the messages that have to be sent into signals that can be transmitted through wires, optical fibres and interplanetary space. A telecommunication system, whether electronic or only electrical consists of some basic blocks or components. Sending messages In telecommunications, a message is transferred along a channel, between a transmitter, i.e. the source, and a receiver, i.e. the destination. A transducer, e.g. a microphone, placed in the transmitter takes the message from the source, e.g. your voice, and translates it into a suitable form which can be transmitted along the communication channel. At the other end, in the receiver, another transducer, e.g. an earphone, delivers a copy of the original message to the destination, e.g. your ear. The basic structure may be implemented by other components. An amplifier can be used to boost the strength of the signal either at the source or at destination; an encoder may be employed in the transmitter and a decoder in the receiver, to put the signal in the right form for transmission. In fact, before sending the information, the message has to be converted into electrical energy either in the analogue, i.e. continuous, or pulse, i.e. discrete or digital form, in order to be processed by electronic circuits. Types of telecommunication systems There are different types of telecommunication systems: Simplex: a one-way system in which the message can travel only in one direction for a simple operation, e.g. using a megaphone; Duplex: a two-way system which works both as a transmitter and a receiver, or a transceiver. When the two directions are used one at a time, we have a half-duplex system as in the case of a walkie-talkie, while telecommunications over the phone are an example of a full-duplex system, i.e. in both directions at the same time through a bi-dimensional transmission channel; Broadcast: a system with one powerful transmitter and many lowpower, sensitive receivers, as in radio and television broadcasts. Multiplex: a system in which multiple transmitters and multiple receivers cooperate and share the same physical channel, as in videoconferencing.